|Publication number||US8020453 B2|
|Application number||US 11/721,769|
|Publication date||Sep 20, 2011|
|Filing date||Dec 14, 2005|
|Priority date||Dec 14, 2004|
|Also published as||EP1825224A1, EP1825224B1, US20090039874, WO2006064236A1|
|Publication number||11721769, 721769, PCT/2005/4835, PCT/GB/2005/004835, PCT/GB/2005/04835, PCT/GB/5/004835, PCT/GB/5/04835, PCT/GB2005/004835, PCT/GB2005/04835, PCT/GB2005004835, PCT/GB200504835, PCT/GB5/004835, PCT/GB5/04835, PCT/GB5004835, PCT/GB504835, US 8020453 B2, US 8020453B2, US-B2-8020453, US8020453 B2, US8020453B2|
|Inventors||Darran Kreit, Mark Anthony Howard|
|Original Assignee||Darran Kreit, Mark Anthony Howard|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (12), Referenced by (8), Classifications (5), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a national phase of International Application No. PCT/GB2005/004835 filed Dec. 14, 2005 and published in the English language.
This invention relates to an inductive displacement detector, operable to measure the displacement of relatively moveable bodies.
Various forms of detector have been used to measure the position of relatively moveable bodies. They are variously referred to as detectors, sensors, transducers, encoders, measurement devices or systems.
A common form of detector is the linearly variable differential transformer (LVDT). Typically, in such devices, a magnetically permeable core moves relative to a primary and secondary winding. Linear forms are typically referred to as LVDTs and rotary forms are variously referred to as rotary variable differential transformers (RVDTs), synchros and resolvers. The degree of electrical coupling between the windings varies in proportion to the position of the core. Such transformers have been used for many years and have a well-deserved reputation for accuracy and reliability. They are widely used in industrial and metrology applications. The transformer construction typically requires precision winding of large numbers of fine conductors. Consequently, they are expensive to produce, delicate and heavy. Such attributes prevent their widespread use.
In order to make such transformer constructions less expensive, more robust and lighter, Inductosyn Ltd. of Stockport, United Kingdom produced various products, dating from the 1960's, using planar forms of linear and rotary transformers. In these, a first, planar, serpentine winding is energised with an AC signal and moves relative to a second, planar, serpentine winding. The degree of electrical coupling is indicative of the relative position of the two windings. Both sets of windings require an electrical connection. This greatly limits the scope for Inductosyns to those applications where electrical connections could be maintained, for example, in low speed linear displacements, rotary encoders with low rotations between 0 and 360 degrees or where slip rings are permissible.
U.S. Pat. No. 4,737,698 discloses an inductive sensor in which a conductive target or electrical intermediate device such as a copper disk moves relative to a planar arrangement of transmit and receive windings. Application of a high frequency input to the transmit winding results in a modulated output from the receive windings which may be demodulated to provide a signal indicative of the electrical intermediate device's position relative to the receive windings. In order to produce sufficiently high signal to noise ratios, a relatively high input voltage signal is required and the target's stand off distance must be kept to a minimum. High input energy produces high electromagnetic emissions and the minimal stand-off distance limits the scope of application. Further, the position of only one target may be detected per set of transmit and receive windings.
U.S. Pat. No. 5,796,250 discloses a rotary displacement encoder in which various rotors each contain an electrically resonant, passive circuit. The rotors are housed in a 3-dimensional arrangement of transmit and receive windings. The invention solves some of the problems normally associated with simple conductive electrical intermediate devices but the winding arrangement is only practical for rotary or odometer types of construction.
The authors have previously disclosed a number of inventions relating to the measurement of electrical intermediate devices which move relative to planar arrangements of transmit and receive windings. For example, GB02/01204 discloses a method in which high frequency carrier signals are modulated with lower frequency signals in order to provide a digital signal generation and processing technique suitable for position measurement. The necessary electronics circuit is expensive; the software is complex; the measurement frequency is limited by the lower modulation frequency and the range of geometries is limited.
The present invention encompasses the concept of a low cost, high speed, accurate and robust device to detect the relative positions of two or more bodies which is applicable to a variety of topologies.
In a preferred embodiment, the device comprises an arrangement of transmit and receive windings attached to a first body and at least one electrical intermediate device attached to a second body operable to move in an axis relative to the first body and wherein the electrical intermediate device comprises a capacitor and inductor in electrical series and wherein the dimension of the inductor measured transversely to the displacement axis varies such that the level of mutual inductance between the transmit and receive windings varies according to the position of the two bodies.
In its broadest aspect, the invention provides a device for measuring the position of a first body relative to a second body comprising: a first body which further comprises an arrangement of transmit and receive windings and a second body which comprises an electrically resonant intermediate device whose inductor width, measured at right angles to the measurement axis, varies such that the level of mutual inductance between the transmit and receive windings varies according to the position of the two bodies.
Viewed from a further aspect, the invention provides an inductive displacement detector according to claim 1.
Preferably the electrically intermediate device comprises a capacitor and inductor in electrical series so as to form a resonant or tank circuit.
Preferably the electrical intermediate device's inductor is formed by conductive tracks on a printed circuit board.
Preferably the detector is controlled by an electronics circuit which is constructed so that it may control several sets of detectors each of which has its own distinct resonant frequency.
Preferably the detector is controlled by an electronics circuit which is constructed so that it may control several sets of detectors each of which has its own period of time in which to transmit and receive signals.
Preferably the electronics circuit comprises a device with electronic memory which may store data such as measurement parameters, configuration data or calibration values.
Preferably the detector is calibrated prior to use with a measuring instrument of higher measurement accuracy and the resulting calibration values stored in the detector's electronic memory.
Preferably the electrical intermediate device comprises windings arranged as a sinusoid over the displacement range.
Preferably the electrical intermediate device comprises windings in a constant coupling loop.
Preferably the electrical intermediate device comprises windings arranged as a repetitive pattern over the displacement range.
Preferably the electrical intermediate device comprises windings arranged as a single coarse and a repetitive fine pattern.
Preferably the electrical intermediate device comprises windings arranged as a multiplicity of repeating patterns of differing pitch so as to form a Vernier style of pattern.
Preferably the electrical intermediate device comprises windings arranged as a multiplicity of repeating patterns, at least two of differing pitch so as to form a Vernier style of pattern and at least one further winding of a smaller repeating pitch so as to form a fine pitch.
Preferably the antenna comprises electrically balanced transmit and receive windings
Preferably the electrical connections to the antennae or electronics are transmitted via a set of transformer windings.
Preferably the electrical intermediate device's electrical circuit contains a switch in series with the capacitor and inductor so as to provide contactless transmission of switch status.
Preferably the electrical intermediate device contains a multiplicity of resonant circuits whose frequency or position or both may be detected so as to provide a method of identification.
Preferably an electrical intermediate device whose position is fixed or known is used together with at least one electrical intermediate device whose position is to be measured so that the reliable operation of the detector may be checked.
Preferably one portion of the electrical intermediate device's inductor has a magnetically permeable core which is operable to displace relative to the inductor's windings so that the change in the core's position may be contactlessly detected as a change in the electrical intermediate device's resonant frequency.
Preferably electrical intermediate devices of various frequencies may be energised by a single antenna so that the position of multiple electrical intermediate devices may be measured by a single antenna.
Preferably the detector is used to measure the relative twist of two parts of a stationary or rotating shaft as a means of measuring the torque applied to the shaft.
Preferably the detector compensates for temperature by using a system to measure temperature and alter the measured position accordingly.
Preferably the detector measures temperature by measuring the resistance of at least one of the antenna's windings.
In the accompanying drawings;
Measurement resolution can be improved by the use of multiple pitch windings as shown in
One draw back of multi-pitch arrangements is that the measured position is ambiguous rather than absolute. Absolute position measurement can be achieved by the use of a second, coarse pitch arrangement extending over the full scale as shown in
A first alternative scheme to coarse and fine pitch winding arrangements is the use of a Vernier technique. A schematic of such an arrangement is shown in
A second alternative to enable high resolution measurement over extended scales can be achieved with the use of a reed or Hall switch for example. In such an instance a magnet is attached to the electrical intermediate device  which triggers the reed or Hall switch to signify that, for example, a second area of the electrical intermediate device  is in operation.
There is a third alternative to enable high resolution measurement over extended scales. This can be achieved with the use of a repetitive winding pitch over a long distance with fundamentally incremental position measurement but wherein the electronic circuit  counts the number of cycles so as to provide an absolute signal. This count can be checked against a shorter electrical intermediate device at a second frequency placed along the multiplicity of repetitive windings. Periodically when the antenna  and electronics circuit  passes over the shorter electrical intermediate device the count may be checked and rectified in software if necessary.
The overall winding pattern may be extended beyond a single period L, at both ends, to improve the linearity at the ends of a linear device.
The shape of the width variation of the circuits on the electrical intermediate device need not be sinusoidal. The width variation may be triangular, circular or other such shape.
Other higher harmonic components may be added to the sinusoidally varying part of the electrical intermediate device to improve linearity. This may be necessary, for example, to account for the disturbing effect of a metal part in the detector's near field.
A further advantage of such transformer constructions is that the antenna  may move freely without trailing electrical wires and connections. For example, both an antenna  and corresponding electrical intermediate device  may be arranged on a rotating shaft to measure the twist between two points of a shaft as a way of measuring applied torque. Displacement of the electrical intermediate device  may be detected by the antenna  which is powered via a transformer [8 & 11]. Preferably, the transformer's primary windings  are on a stationary part of the assembly and the secondary windings  are on the rotating parts of the assembly. The received signals in the antenna  are sent back to the host electronics  via the transformer [8 & 11]. Such arrangements are particularly useful in steering columns where applied torque and steering may be measured. In order that the shaft twists a sufficiently large distance to be measured it can be slotted. Small angular twists are preferably measured using a multi-pitch arrangement of antenna  and electrical intermediate device .
The pitch of the receive windings is not necessarily L/4 for an electrical intermediate device of winding pitch L. This dimension was described for reasons of simplicity so as to make clear the possible use of simple arctan calculations performed on the two received signals.
A portion of the electrical intermediate device  is shown to vary as a smooth sinusoid however the variation need be neither smooth nor sinusoidal. For example, the function can be approximated by using largely rectangular or nested rectangular loops. In addition it will be obvious that any of the antenna  or electrical intermediate device  windings can be formed using multiple turns of generally the same shape to maximise electromagnetic coupling.
The antenna's transmit and receive windings [2 a, 2 b, 2 c & 2 d] may be simplified when the invention is used in a pulse echo mode. In this mode the windings are first of all energised with an AC signal which is then switched off to allow the windings then to receive any signals coming back from a resonating electrical intermediate device. Only 2 windings, with known separation, are required and there is no requirement necessarily for electrical balancing.
There is no absolute size limitation to the invention. The limits are set by limits of manufacturing processes rather than physical laws. At one extreme, very large sensors can be produced by winding copper wire over pegs arranged at defined positions on a surface. At the other extreme, very small sensors can be produced using deposition of conductive tracks on a silicon wafer. This is particularly advantageous when the tracks are deposited on the same silicon as that used for the electronics circuit in the form of an application specific integrated circuit. The use of conductive inks printed on to an insulating substrate such as polyester or polyamide are particularly useful technique to produce electrical intermediate devices and antennae. Advantageously, polyester and polyamide substrates may be produced in a flexible form which may also be printed with a contact adhesive for ease of attachment to a host system. In printed constructions electrical insulation at cross over points can be maintained by printing a first conductive track followed by an insulating layer at the area of the cross over and then another conductive track over the top of the insulating layer. Glass is a particularly good substrate in harsh environments due to its stability and low coefficient of thermal expansion. Further construction methods include double sided printed circuit board with plated through holes; ultrasonic bonding of insulated copper wire on to an insulated substrate and windings which are laser cut or stamped and then folded from sheet metal such as copper, aluminium or steel.
As has already been stated in the description of
To a significant extent, variation in the position of the electrical intermediate device  relative to the antenna  in axes other than the main measurement axis does not affect the measured value. In particular, the stand off distance between electrical intermediate device  and antenna  in the z-axis can vary without altering the measured displacement. The range of acceptable variation can be extended by bracketing the amplification factors used in the electronics circuit according to the amplitude of the received signals. If the electrical intermediate device  to antenna  distance is large then the amplitude of received signals will be small and large amplifications should be applied. The converse applies if electrical intermediate device  to antenna  distance is small.
The variation in signal amplitude caused by variation in stand off distance can be used as a relatively coarse measurement in multi-axis arrangements.
The invention is able to identify a multiplicity of electrical intermediate devices  and measure their displacement relative to the antenna  in a roughly concurrent fashion. This is accomplished by providing each electrical intermediate device  with its own resonant frequency. Individual resonant frequencies are most readily attained by careful selection of different capacitor values for example to produce 3 electrical intermediate devices  with resonant frequencies of 1, 2 and 3 MHz. The electronics circuit  can be programmed to excite at these frequencies in turn and carry out measurements for each electrical intermediate device . In order to maximise the frequency and accuracy of measurements more sophisticated excitation and measurement algorithms can be used where, for example, the electrical intermediate device  which is found to be generally stationary is measured least and the one that has moved most recently or most often is measured most frequently.
Two transmit windings can be used rather than one with two loops [2 a & 2 b] thus avoiding the requirement for the constant width loops [1 b & 1 c] of the electrical intermediate device  windings. Each transmit winding will overlap the variable width portion of the electrical intermediate device [1 a & 1 d]. In such an arrangement, the electronics circuit  switches between transmit circuits and chooses the one which produces maximum receive signals. In this case the invention's level of electromagnetic emissions can be reduced by balancing each loop of each transmit winding with a counter wound loop away from the electrical intermediate device . In this way any transmissions to the far field will be negated. This also maintains balance with each receive windings [2 c & 2 d].
It will be appreciated by those skilled in the art that the invention is not restricted by a particular number or arrangement of windings and that various permutations of number, spacing and arrangement are feasible. For example, a repeating but aperiodic winding arrangement of the electrical intermediate device's inductor  may be used with a single transmit winding and three receive windings spaced along the measurement path. Electronic comparison of the amplitude of the received signals in each of the receive windings will be unique for any position of the electrical intermediate device  relative to the antenna . Such readings may be compared to a previously made look-up table held in electronic memory in order that a unique or absolute, rather than incremental, position may be reported.
Multiple electrical intermediate devices  and multiple antennae  may be constructed on the same physical unit of printed circuit board by simply avoiding electrical connection between the various systems. Multiple layer PCB is particularly useful in this regard. Such constructions are particularly advantageous in detectors for safety related environments where electrical redundancy is necessary. In an electrically redundant system a multiplicity of electrical intermediate devices  of varying frequencies may be concurrently detected using a multiplicity of antennae  energised with the relevant frequencies, each antenna  being controlled by its own electronic circuit .
In some safety related applications or applications where high levels of availability and reliability are required then detectors may be constructed with self checking hardware and software. Not only can the electronic circuit's  software contain the traditional techniques associated with checking for open circuits, out of bounds measurements etc. but an electrical intermediate device  may be fixed relative to the antenna . In this way a self diagnostic check may be carried out by measuring the position of the fixed electrical intermediate device . If the fixed device  appears to have moved form its known position past predefined limits then this may be taken as a fault and an alarm or corrective action taken accordingly.
In order to maximise received signal strength and hence maximise the signal:noise ratio the electronics circuit  should include a frequency tuning circuit so that the exact resonant frequency of the electrical intermediate device  or devices may be used as the transmit frequency. In this way a maximum amount of transmitted energy goes in to producing electrical resonance of the electrical intermediate device  and, in turn, the electrical intermediate device  produces maximum electromagnetic signal.
The invention's susceptibility to electromagnetic emissions can be improved by balancing each loop of the receive circuit [2 c & 2 d] with a counter wound loop away from the electrical intermediate device . In this way any incoming emissions from the far field will be negated before it is detected by the electronics circuit .
Preferably any material present between the electrical intermediate device  and antenna  is an insulator such as plastic, ceramic or wood. A metal barrier can be placed between them so long as the excitation or resonant frequency is sufficiently low to permit the signals to carry through the metal's skin depth. If a metal barrier is essential then preferably the metal has a relatively low magnetic permeability such as non-magnetic stainless steel (e.g. 316 grade stainless). A frequency of 40 kHz is, for example, sufficient to permit the transmission of signals through 2 mm thick, non-magnetic 316 grade stainless steel sheet.
Thus far the antenna  has been described with co-planar transmit and receive windings [2 a, 2 b, 2 c & 2 d]. This is preferable but not necessary. The various windings may be, for example, placed on either side of the electrical intermediate device and act as a mechanical guide.
The detector has particular utility in determining the position of vehicles and automatically guided vehicles. In such applications the electrical intermediate device  is preferably fixed to the floor and the detector's antenna  and control electronics  attached to the underside of the vehicle (or vice versa) so that a position reading may be taken as the vehicle passes over it.
For most applications the effect of extreme or changing temperatures will have negligible effect on measurement performance. In some applications, however, very high accuracy measurement is required even though the operating temperature range or variation rate may be extreme. In such instances, the relatively small expansion or contraction of the antenna  or target  may lead to erroneous measurement. Such temperature effects may be counteracted by measuring the actual operating temperature and modifying the measured position accordingly, i.e. reducing or increasing the measured value according to the temperature. Temperature can be measured using a thermocouple or resistance device but preferably the resistance of one or more of the antenna windings [1 a, 1 b or 1 c] can be used to provide an indication of temperature. Measurement of the winding resistance is preferable to measurement by a thermocouple because the windings provide a more representative measurement due to their position along the measurement axis (rather than being constrained to a single point as with a thermocouple).
There are many applications for the invention including, but not limited to: actuators, aileron controls, angle sensors, radar antenna tracking, anti-counterfeit devices, audio controls, automatic guided vehicles, automatic teller machines, automation equipment, ball screws, boilers, brake sensors, brake wear sensors, burners, climate controls, cockpit controls, component identification, consumer electronics, cookers, cooking ranges, cooktops, dials, direction indicators, dishwashers, displacement sensors, door travel sensors, elevators, end of shaft encoders, fitness equipment, flow sensors, food mixers, fuel level sensors, fuel metering, games, gauges, giant magnetoresistive sensor replacements, guided vehicle tracking, gunnery sights, Hall affect replacements, headlamp level controls, HVAC sensors, hydraulic actuators, hydraulic valves, identification tags, impellers, inclinometers, indexing tables, indicator gauges, Inductosyn replacements, industrial control panels, joysticks, kitchen goods, lifts, lighting controls, limit switch replacements, linear actuators, liquid level sensors, load sensors, LVDT replacements, machine tools, magnetostrictive sensor replacements, marine engines, marine equipment, mining equipment, missile guidance, motion controllers, motor encoders, odometers, packaging equipment, palletisers, paper thickness sensors, pedal sensors, pen sensing, petrochemical sensors, plotter controls, pneumatic actuators, pneumatic valves, pressure sensors, printer write heads, PRNDL sensors, proximity sensors, push buttons, radar controls, ride height sensors, robots, roll/pitch/yaw sensors, roller separation sensors, rotary encoders, RVDT replacements, safety switches, seating instrumentation, security tags, servo motors, shaft encoders, sheet feeders, skis, sliders, speed sensors, sports equipment, steering angle sensor, steering column controls, stepper motors, strain measurement, suspension dampers, suspension sensors, tachometers, tamper evident devices, throttle controls, tilt sensors, torque sensors, toys, traction control, transmission sensors, turbines, user interface elements, utility meters, valves, velocity sensors, vibration sensors, washing machines, weight sensors, wheel sensors, workpiece identification.
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|International Classification||G01D5/20, G01L3/00|